Earth-boring bits, such as roller cone rock bits and percussion rock bits, may be employed for drilling oil wells through rock formations, or for drilling blast holes for blasting in mines and construction projects. Earth-boring bits also are referred to as drill bits. During operation, a drill bit is connected to the end of a drill string and rotated to drill through the earth. One variety of drill bits, the roller cone rock bits, have a plurality of wear-resistant inserts secured in rotatable cones attached to a bit body. The inserts usually have a substantially cylindrical body portion which is adapted to fit in a cylindrical hole in the roller cone and a top portion which protrudes from the roller cone for contacting an earthen formation.
When a roller cone rock bit is used to drill a borehole, it is important that the diameter or "gage" of the borehole be maintained at a desired value. The first row of inserts from the center of the rock bit on each roller cone that cuts to a full gage borehole typically is referred to as the "gage row." This row of inserts generally is subjected to the greatest wear, as it both reams the borehole wall and cuts the corner of the borehole. As the gage row inserts wear, the diameter of the borehole being drilled may decrease below the original gage diameter of the rock bit. When the bit is worn out and removed, the diameter of the bottom portion of the hole may be less than the gage diameter, or "under-gage." When the next bit is run in the hole, it is required to ream that bottom portion of the borehole to bring it to the full gage diameter. This not only takes substantial time but also adds to the wear on the gage row inserts of the next bit. This additional wear on the gage row inserts may result in an increased length of under-gage borehole as the bit wears out.
In addition to gage row inserts, a conventional bit typically includes a number of inner row inserts located on a roller cone and disposed radially inward from the gage row. These inner row inserts are sized and configured for cutting the bottom of the borehole. Sometimes, a conventional bit also may include a plurality of secondary inserts located between the gage row inserts. These inserts, referred to as "nestled gage inserts," typically cut the full gage of the borehole and also assist the gage inserts in cutting the borehole corner. Moreover, a conventional rock bit may further include a row of heel inserts located on the frustoconical surface of a roller cone. The heel row inserts generally scrape and ream the side wall of a borehole as the roller cone rotates about its rotational axis.
The performance of a rock bit is measured, in part, by total drilling footage and rate of penetration. As the inserts on a rock bit wear, the rate of penetration typically decreases. When the inserts have been substantially worn out, it is no longer economical to continue drilling that bit, and the bit is replaced. The amount of time required to make a "round trip" for replacing a bit, i.e., pull all of the drill string out of the borehole, replace the worn-out bit, and reassemble the drill string into the borehole, essentially represents time lost from actual drilling. This time can become a significant portion of the total time for completing a well. Therefore, it is highly desirable to design and manufacture inserts that would increase the rate of penetration and total drilling footage of a rock bit. In particular, there have been numerous attempts to reduce wear on the gage row inserts to increase the length of a borehole drilled to full gage.
Two kinds of wear-resistant inserts typically are used in a rock bit--tungsten carbide inserts and polycrystalline diamond ("PCD") enhanced inserts. Tungsten carbide inserts are formed of cemented tungsten carbide. A typical composition for cemented tungsten carbide is tungsten carbide particles dispersed in a cobalt binder matrix. The PCD enhanced insert, an improvement over the tungsten carbide insert, typically includes a cemented tungsten carbide body as a substrate and a layer of polycrystalline diamond directly bonded to the tungsten carbide substrate on the top portion of the insert.
Although the polycrystalline diamond layer is extremely hard and wear-resistant, a PCD enhanced insert still may fail during normal operation. The typical failure mode is cracking of the polycrystalline diamond layer due to high contact stress, lack of toughness, and insufficient fatigue strength. A crack in the polycrystalline diamond layer during drilling may cause the polycrystalline diamond layer to spall or delaminate. Furthermore, a crack in the polycrystalline diamond layer may propagate through the cemented tungsten carbide body of the insert and cause more massive failure of the insert. On the other hand, wear of the polycrystalline diamond layer can be a failure mode leading to failure of an insert, particularly in percussion rock bits.
For the foregoing reasons, there exists a need for PCD enhanced inserts that possess not only high hardness but also desired toughness and other properties to drill through rock formations without premature breakage or delamination of the polycrystalline diamond layer.